ScalingStacks

3.9 [035F]

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3.9

Now let F:Nℝ′→NℝF:N_{\mathbb{R}}^{\prime}\rightarrow N_{\mathbb{R}} be an affine map whose underlying linear map is integral, i.e. induced by a homomorphism N′→NN^{\prime}\rightarrow N. We will define the push-forward of a weighted integral ℝ{\mathbb{R}}-affine polyhedral complex (𝒞′,m)({\mathscr{C}}^{\prime},m) of pure dimension nn on Nℝ′N_{\mathbb{R}}^{\prime}. For details, we refer to [AR10], §7. After a subdivision of 𝒞′{\mathscr{C}}^{\prime}, we may assume that

F∗​(𝒞′):={F⁡(σ′)∣σ′ is a face of ν′∈𝒞′ with dim(F⁡(ν′))=n}F_{*}({\mathscr{C}}^{\prime}):=\{F(\sigma^{\prime})\mid\text{$\sigma^{\prime}$ is a face of $\nu^{\prime}\in{\mathscr{C}}^{\prime}$ with $\dim(F(\nu^{\prime}))=n$}\}

is a polyhedral complex in NℝN_{\mathbb{R}}. We define the multiplicity of an nn-dimensional F⁡(σ′)∈F∗​(𝒞′)F(\sigma^{\prime})\in F_{*}({\mathscr{C}}^{\prime}) by

mF⁡(σ′):=∑ν′∈𝒞n′,ν′⊂F−1​(F⁡(σ′))[Nν′′:NF⁡(σ′)]mν′.m_{F(\sigma^{\prime})}:=\sum_{\nu^{\prime}\in{\mathscr{C}}^{\prime}_{n},\,\nu^{\prime}\subset F^{-1}(F(\sigma^{\prime}))}[N_{\nu^{\prime}}^{\prime}:N_{F(\sigma^{\prime})}]m_{\nu^{\prime}}.

Endowed with these multiplicities, we get a weighted integral ℝ{\mathbb{R}}-affine polyhedral complex F∗​(𝒞′,m)F_{*}({\mathscr{C}}^{\prime},m) of NℝN_{\mathbb{R}}. If (𝒞′,m)({\mathscr{C}}^{\prime},m) is a tropical cycle, then F∗​(𝒞′,m)F_{*}({\mathscr{C}}^{\prime},m) is also a tropical cycle. It might happen that F∗​(𝒞′,m)F_{*}({\mathscr{C}}^{\prime},m) is empty, then we get the tropical zero cycle.

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